# Calculating Horsepower

By Staff
Calculating horsepower

Many times a collector would like to know the horsepower of an unmarked engine. If one knows the bore and stroke, which are easily measured, an estimate of the horsepower rating of an engine can be obtained by using the given chart. Instructions are as follows:

1. Draw a straight line connecting the measured values of bore and stroke, on columns A and B. (An example is the dotted line connecting 5″ diameter bore and stroke of 8″.) The result on column C is the displacement (200 cubic inches).

2. Connect this point (on column C) with the speed of the crankshaft in RPM, given in column D. (In the example, the 200 cubic inch point is connected by the second dotted line with 500 RPM.) The resulting estimated horsepower is given in column E. (Example, 5.5 HP.)

Since the horsepower developed by an engine is directly related to crankshaft RPM, the collector must either measure the RPM for normal operation, or estimate, if the data is not available from markings or literature.

The chart, which is called a nomograph, is based on the formula:

HP = (BORE)2 X STROKE X RPM/18,000

from “Cyclopedia of Engineering,” American Technical Society, 1916, and is only an estimate, derived from averaged measurements (made in early 1900s) on many different engines. The accuracy of the chart is about 10%. Some of the basic assumptions used in developing the nomograph are:

a. Good for 4-cycle engines only.

b. HP is given for one cylinder only; to obtain horsepower of multi-cylinder engines, multiply by the number of cylinders.

c. Low compression gasoline engines only; the thermodynamics and efficiencies of kerosene and diesel engines are different.

Thanks to John Skotko for help with the nomograph scale.

• Published on Sep 1, 1980
Gas Engine Magazine
Preserving the History of Internal Combustion Engines